The receptor tyrosine kinase, c-Met, and its ligand hepatocyte growth factor (HGF) have become leading candidates for targeted cancer therapies.
c-Met is the cell surface receptor for hepatocyte growth factor (HGF), also known as scatter factor. The c-Met receptor is a disulfide-linked heterodimer consisting of extracellular a and β chains. The α chain, heterodimerized to the amino-terminal portion of the β chain, forms the major ligand-binding site in the extra cellular domain. HGF binding induces c-Met receptor homodimerization and phosphorylation of two tyrosine residues (Y1234 and Y1235) within the catalytic site, regulating kinase activity.
HGF-mediated activation of c-Met results in a complex genetic program referred to as “invasive growth”, consisting of a series of physiological processes, including proliferation, invasion, and angiogenesis, that occur under normal physiological conditions during embryonic development and pathologically during oncogenesis. Signalling through c-Met promotes proliferation and cell survival through a variety of downstream effectors.
In tumour cells, c-Met activation causes the triggering of a diverse series of signalling cascades resulting in cell growth, proliferation, invasion and protection from apoptosis. The underlying biological mechanisms for tumorigenicity of c-Met are typically achieved in three different ways: (a) with the establishment of HGF/c-Met autocrine loops; (b) via c-Met or HGF over-expression; and (c) in the presence of kinase-activating mutations in the c-Met receptor coding sequence. HGF and c-Met expression has been observed in tumour biopsies of most solid tumours, and c-Met signalling has been documented in a wide range of human malignancies, including bladder, breast, cervical, colorectal, gastric, head and neck, liver, lung, ovarian, pancreatic, prostrate, renal and thyroid cancers.
Activation of c-Met by its ligand, HGF, can occur in either a paracrine or an autocrine manner. Paracrine activation can become pathological in the presence of abnormal HGF production. Autocrine activation occurs when tumour cells aberrantly express both HGF and its receptor. In addition, c-Met activation can occur in an HGF-independent manner, mediated by c-Met homodimerization.
A wide variety of human malignancies exhibit sustained c-Met stimulation, over-expression or mutation, including carcinomas of the breast, liver, lung, ovary, kidney and thyroid. Activating mutations in c-Met have been positively identified in patients with a particular hereditary form of papillary renal cancer, directly implicating c-Met in human tumorigenesis. Aberrant signalling of the c-Met signalling pathway due to disregulation of the c-Met receptor or over-expression of its ligand, HGF, has been associated with an aggressive phenotype. Extensive evidence that c-Met signalling is involved in the progression and spread of several cancers and an enhanced understanding of its role in disease have generated considerable interest in c-Met and HGF as major targets in cancer drug development (Eder et al, Clin Cancer Research; 15(7); 2009).
A variety of c-Met pathway antagonists with potential clinical applications are currently under clinical investigation. Potential c-Met antagonists include monoclonal antibodies which block the interaction of c-Met with its ligand HGF. The most extensively described is the anti-c-Met 5D5 antibody generated by Genentech (WO96/38557). 5D5 behaves as a potent agonist when added alone in various models and as an antagonist when used as a Fab fragment or a one-armed antibody (MetMab).
WO 2009/007427 describes mouse monoclonal antibodies to c-Met and chimeric variants in which the antigen-binding domains of the mouse monoclonal antibody, or a humanised variant thereof, are coupled to the constant region of human IgG1. However, whilst the original mouse monoclonal antibody, 224G11, exhibits antagonist activity without significant intrinsic agonist activity, coupling of the antigen binding domains of 224G11 to human IgG1 generated a chimeric form of 224G11 which exhibited some agonist activity associated with a reduced antagonist efficacy. The agonist activity exhibited by the chimeric form of 224G11 can be reversed by engineering point mutations in the heavy chain hinge domain of the human IgG1. In this engineered variant several human amino residues in the hinge region are replaced by murine residues occurring at equivalent positions in the murine IgG1 sequence. C-Met receptor antagonist activity is restored in the resulting engineered variant, but the overall structural and sequence homology to human antibodies is reduced as a result of the mutations required in the hinge region. In addition, at least one of the hypervariable loops in 224G11 adopts a canonical structure which is not found in the human antibody repertoire.
WO 2007/126799 describes fully human monoclonal antibodies to c-Met. These antibodies behave as antagonists of the interaction with HGF, but no data is presented regarding the intrinsic agonist activity of these antibodies or their ability to inhibit c-Met dimerization.
WO 2010/059654 also describes monoclonal c-Met antibodies. These antibodies are characterised by binding to the α-chain of human c-Met and inducing internalisation of cell surface human c-Met.